The invention relates to a circuit for suppressing pulse-shaped interferences in an audio signal caused by scratches on a phonograph recording, comprising, arranged between an audio frequency input and an audio frequency output, a signal processing section, provided with a noise suppressor having a control input, as well as a control signal section arranged between the audio frequency input and the control input, the control signal portion comprising a first threshold selection circuit connected to the audio frequency input for selecting pulse-shaped signals from the audio signal.
Such a circuit is disclosed in the article "Audio disc scratch filter" by M. R. Sachs and J. M. Bullingham, published in "Electronics Letters" dated Dec. 9, 1976, vol. 12, no. 25, pages 656-657.
In the prior art circuit the occurrence of noise caused by a scratch on the phonograph record is detected in the first threshold selection circuit by means of a high-pass filter, a level control circuit and a detector.
In some fragments of music recorded on a phonograph record, in which certain musical instruments have the upperhand, such as, for example, percussion instruments, and brass wind instruments, there are, however, pulse-shaped music signals which are very similar to noise caused by scratches on the phonograph record. These pulse-shaped music signals follow one another in a fast rhythm during such a fragment of music and the detector in the first threshold selection circuit recognizes them as a rapid, continuous succession of interferences. Consequently the noise suppressor may be actuated consecutively and rapidly, which may result in an annoying distortion of the output signal of the circuit.